What Is the Limbic System and What Is Its Function?

The limbic system is a set of brain structures involved in the control of emotions, long-term memory, behavior, and olfaction [1]. It is made of the following structures:

• The limbic Lobe
• Hippocampal Formation
• The Amygdala
• The Septal Area
• The Hypothalamus

1. What Is the Function of the Limbic Lobe?

The limbic lobe is a vital region of the brain involved in emotion, learning, memory, and motivation [2]. It is a C-shaped brain region that includes:

• Cingulate gyrus
• Parahippocampal gyrus
• Subcallosal Gyrus

1.1. What Is Cingulate Gyrus and Its Function?

The cingulate gyrus is a brain structure located toward the middle (medial aspect) of the cerebral hemisphere and is involved in the formation and processing of emotions, memory, and learning.

1.2. What Is Parahippocampal Gyrus and Its Function?

The parahippocampal gyrus is a brain structure located toward the middle of the temporal lobe and is involved in-memory processing, storage, and recall.

1.3. What Is Subcallosal Gyrus and Its Function?

The subcallosal gyrus is a thin layer (lamina) located towards the middle of the hemisphere involved in the regulation of stress responses by suppressing the amygdala’s responsiveness to fear.

2. What Is Hippocampal Formation?

The hippocampal formation is located towards the middle of the temporal lobe of the brain and is made of 4 structures [3]:

• The Dentate Gyrus
• The Hippocampus Proper
• The Subiculum Complex
• The Entorhinal Cortex

2.1. What Is Dentate Gyrus?

The dentate gyrus is a structure located toward the middle of the temporal lobe that receives fibers from the entorhinal cortex and may be involved in episodic memory (Type of long-term memory).

2.2. What is Hippocampus?

The hippocampus proper is the structure of the brain where episodic memories are formed and stored.

In addition to its role in long-term memory, the hippocampus is also involved in learning, flexible and goal-oriented behavior, navigation, and spatial orientation.

2.3. What Is Subiculum Complex and Its Function?

The subiculum complex is a structure located toward the middle of the temporal lobe involved in working memory and drug addiction.

2.4. What Is Entorhinal Cortex and Its Function?

The entorhinal cortex is a structure located toward the middle of the temporal lobe which connects the hippocampus and the neocortex and is involved in navigation, perception of time, and memory formation, consolidation, and optimization during sleep.

3. What Is the Amygdala and What It Controls?

The Amygdala is an almond-shaped structure located toward the middle of the temporal lobe involved in emotional response (fear, anxiety, and anger), decision making, and memory (emotional and fear associated memories) [4].

4. What Is Septal Area and Its function?

The septal area is a structure located in the inferior and posterior part of the middle of the frontal lobe involved in emotional, motivational, and spatial behavior through its nuclei connections with the hypothalamus, amygdala, olfactory bulb, cingulate gyrus, thalamus, habenula, and midbrain.

5. What Is the Hypothalamus and What It Controls?

The hypothalamus is a structure located in the diencephalon below the thalamus and is involved in functional linking between the nervous system and the endocrine system [5].

It is divided into three regions, the anterior region (supraoptic), the middle region (tuberal), and the posterior region (mammillary), and each region is subdivided into areas containing specialized nuclei.

The anterior region has 3 areas known as preoptic, medial, and lateral, involved in thermoregulation, circadian rhythms (suprachiasmatic nucleus), and endocrine secretion of hypothalamic hormones (e.g., corticotropin-releasing hormone).

The meddle region (tuberal) has 2 areas, medial and lateral, involved in the control of blood pressure, heart rate, gastrointestinal stimulation, hunger, secretion of the growth hormone-releasing hormone (GHRH), and neuroendocrine control.

The posterior region has also 2 areas, medial and lateral, involved in memory, learning, arousal, sleep, shivering, pupillary dilatation, and energy balance.

6. What Happens If the Limbic System Is Damaged?

Damage in the structures of the limbic system can result in the following disorders [1]:

• Epilepsy
• Dementia
• Limbic Encephalitis
• Schizophrenia
• Anxiety Disorders
• Affective Disorders
• ADHD
• Korsakoff’s Psychosis
• Klüver-Bucy Syndrome
• Autism

6.1. Epilepsy

Hippocampal sclerosis (tissue stiffening) can result in temporal lobe epilepsy which can be further complicated when other structures, including the amygdala and the parahippocampal gyrus, are also involved (Mesial Temporal Sclerosis).

6.2. Dementia

Alzheimer’s’ disease is associated with senile plaques and neurofibrillary tangles affecting the hippocampus and the dentate gyrus.

6.3. Limbic Encephalitis

Limbic encephalitis is an inflammation that affects parts of the limbic structures due to an autoimmune disorder (non-paraneoplastic limbic encephalitis) or a tumor (Paraneoplastic limbic encephalitis).

6.4. Schizophrenia

Schizophrenia is a complex and severe mental health disorder that is classified as a type of psychosis. It is characterized by significant alterations in thoughts, perception, mood, and behaviors.

The causes are not well known; however, it was suggested that the causes may be associated with a cognitive decline due to alterations in brain development, particularly of the cerebral cortex.

The reduced volumes of the limbic system structures including the hippocampus and the entorhinal cortex in schizophrenia patients may be one of the causes of this disorder.

6.5. Anxiety Disorders

Failure of the hippocampus and the cingulate gyrus in modulating the activity of the amygdala can result in anxiety disorders.

6.6. Affective Disorders

Variation in the volumes of the frontal lobes, amygdala, hippocampus, and basal ganglia, have been associated with affective disorders. 

6.7. ADHD

Loss of connections between the orbitofrontal cortex and the amygdala may explain the disinhibition behavior observed in ADHD individuals.

6.8. Korsakoff’s Psychosis

Korsakoff’s psychosis is caused by damage to mammillary bodies, the thalamus (e.g., tumors, ischemia, or stroke), and hypothalamus resulting in amnesia, the deficit in explicit memory, and confabulation.

6.9. Klüver-Bucy Syndrome

This disease is associated with symptoms including hypersexuality, hyperphagia, placidity, visual agnosia, and hyperorality. It is due to bilateral lesions affecting the amygdala and the inferior prefrontal cortex.

6.10. Autism

Alterations in limbic structures, including the amygdala and the cingulate gyrus, have been associated with autism and Asperger’s syndrome.

Conclusion

Although its involvement in the control of emotions, long-term memory, behavior, and olfaction is known, the complexity of the limbic system makes its clinical study difficult.

However, advances in behavioral studies, functional magnetic resonance imaging (MRI), deep brain stimulation, and perfusion studies, have allowed a better understanding of this system, and future functional imaging techniques will certainly uncover more hidden functions of this fascinating part of the brain.